Security researchers at Qualys have uncovered a nine-year-old vulnerability in the XFS filesystem that enables local unprivileged users to overwrite protected files and gain root privileges. The RefluXFS vulnerability, tracked as CVE-2026-64600, has been present in Linux systems since version 4.11 released in 2017 and affects all stable kernel versions released since then.
The security flaw impacts systems running XFS with the reflink feature enabled, specifically when protected files and attacker-accessible directories reside on the same filesystem. This configuration is default in Red Hat Enterprise Linux, CentOS Stream, Oracle Linux, Rocky Linux, AlmaLinux, CloudLinux, Fedora Server, and Amazon Linux. According to researchers, the vulnerability potentially affects more than 16.4 million systems worldwide.
Which Systems Are Vulnerable to RefluXFS?
The vulnerability does not pose risks to RHEL 7 systems, as they predate XFS reflink support. Additionally, Debian, Ubuntu, SLES, and openSUSE typically do not use XFS as the root filesystem by default, reducing their exposure. The bug specifically targets configurations where XFS reflink functionality is active and operational.
How Does the RefluXFS Attack Work?
Exploitation requires an attacker to create a reflink clone of a protected file, such as /etc/passwd or the SUID binary /usr/bin/su, then launch multiple parallel direct write operations using O_DIRECT. During reflink cloning, the original and copy initially reference the same disk sectors. When the attacker attempts to modify the copy, XFS must allocate separate space. However, parallel requests trigger a race condition where one request uses outdated information, causing it to overwrite disk sectors belonging to the original protected file.
The attack modifies the protected file’s content while the attacker works only with their copy. Critically, the protected file’s metadata remains unchanged, preserving owner, access permissions, timestamps, and SUID bits. Only the file content changes. These modifications write directly to disk, survive system reboots, and leave no traces in kernel logs. Researchers report that in testing environments, they typically won the race condition in less than ten seconds.
Can Standard Security Mechanisms Stop This Attack?
Standard protective mechanisms prove ineffective against this vulnerability. SELinux, seccomp, kernel lockdown, container isolation, KASLR, SMEP, and SMAP cannot prevent the attack because the issue stems from XFS handling of reflink copies. The attack writes bypass the protected file’s inode, modifying content directly on disk without exploiting memory handling errors.
How Was RefluXFS Discovered?
Qualys researchers discovered the vulnerability through collaboration with Anthropic as part of Project Glasswing, gaining access to the closed AI model Claude Mythos Preview. The AI received instructions to find Linux kernel vulnerabilities similar to Dirty COW involving race conditions. The model identified the XFS problem and produced a working proof-of-concept exploit. Researchers verified the model’s reasoning, confirmed technical details, and successfully reproduced the attack.
The fix entered the kernel source code on July 16, 2026, and distribution vendors have begun releasing patches. Administrators should update kernels immediately, particularly on public and multi-user systems.
Source: Xakep